Lightning Detection And Ranging Research Articles

Radar Nowcasting of Total Lightning over the Kennedy Space Center

Abstract A long-term radar dataset over Melbourne, Florida, was matched with three-dimensional lightning data to optimize radar-derived predictors of total lightning over the Kennedy Space Center (KSC). Four years (2006–09) of summer (June–August) daytime (about 1400–0000 UTC) Weather Surveillance Radar-1988 Doppler data were analyzed. Convective cells were tracked using a modified version of the Storm Cell Identification and Tracking (SCIT) algorithm, and correlated to cloud-to-ground (CG) lightning data from the National Lightning Detection Network (NLDN) and grouped intracloud (IC) flash data from the KSC Lightning Detection and Ranging (LDAR) I and II networks. Reflectivity values at isothermal levels and a vertically integrated ice (VII) product were used to optimize radar-based forecasting of both IC and CG lightning. Results indicate the best reflectivity threshold predictors of CG and IC lightning according to the critical success index (CSI) were 25 dBZ at −20°C and 25 dBZ at −15°C, respectively. The best VII predictors of CG and IC were the 30th (0.840 kg m−2) and 5th percentiles (0.143 kg m−2), respectively, suggesting less ice mass is needed in the main mixed-phase region for IC flashes to occur. In addition, VII at lightning initiation (both CG and IC) was higher than at cessation. Seventy-six percent of cells had IC initiation before CG initiation. Using the first IC flash as a predictor of CG occurrence also statistically outperformed other CG predictors, but yielded a 2.4-min average lead time. However, this lead time is comparable to the reflectivity threshold and VII methods when accounting for radar scan and processing time.

Developing empirical lightning cessation forecast guidance for the Cape Canaveral Air Force Station and Kennedy Space Center

This research addresses the 45th Weather Squadron's (45WS) need for improved guidance regarding lightning cessation at Cape Canaveral Air Force Station and Kennedy Space Center (KSC). KSC's Lightning Detection and Ranging (LDAR) network was the primary observational tool to investigate both cloud‐to‐ground and intracloud lightning. Five statistical and empirical schemes were created from LDAR, sounding, and radar parameters derived from 116 storms. Four of the five schemes were unsuitable for operational use since lightning advisories would be canceled prematurely, leading to safety risks to personnel. These include a correlation and regression tree analysis, three variants of multiple linear regression, event time trending, and the time delay between the greatest height of the maximum dBZ value to the last flash. These schemes failed to adequately forecast the maximum interval, the greatest time between any two flashes in the storm. The majority of storms had a maximum interval less than 10 min, which biased the schemes toward small values. Success was achieved with the percentile method (PM) by separating the maximum interval into percentiles for the 100 dependent storms. PM provides additional confidence to the 45WS forecasters, and a modified version was incorporated into their forecast procedures starting in the summer of 2008. This inclusion has resulted in ∼5–10 min time savings. Last, an experimental regression variant scheme using non‐real‐time predictors produced precise results but prematurely ended advisories. This precision suggests that obtaining these parameters in real time may provide useful added information to the PM scheme.

Small negative cloud‐to‐ground lightning reports at the NASA Kennedy Space Center and Air Force Eastern Range

The NASA Kennedy Space Center (KSC) and Air Force Eastern Range (ER) use data from two cloud‐to‐ground (CG) lightning detection networks, the Cloud‐to‐Ground Lightning Surveillance System (CGLSS) and the U.S. National Lightning Detection Network™ (NLDN), and a volumetric lightning mapping array, the Lightning Detection and Ranging (LDAR) system, to monitor and characterize lightning that is potentially hazardous to launch or ground operations. Data obtained from these systems during June–August 2006 have been examined to check the classification of small, negative CGLSS reports that have an estimated peak current, ∣Ip∣ less than 7 kA, and to determine the smallest values of Ip that are produced by first strokes, by subsequent strokes that create a new ground contact (NGC), and by subsequent strokes that remain in a preexisting channel (PEC). The results show that within 20 km of the KSC‐ER, 21% of the low‐amplitude negative CGLSS reports were produced by first strokes, with a minimum Ip of −2.9 kA; 31% were by NGCs, with a minimum Ip of −2.0 kA; and 14% were by PECs, with a minimum Ip of −2.2 kA. The remaining 34% were produced by cloud pulses or lightning events that we were not able to classify.

Evolution of the total lightning structure in a leading‐line, trailing‐stratiform mesoscale convective system over Houston, Texas

Line‐normal, vertical cross sections of Houston Lightning Detection and Ranging (LDAR) VHF radiation sources and radar reflectivity provide new insights into the three‐dimensional total lightning structure and evolution of a leading‐line, trailing‐stratiform (LLTS) mesoscale convective system (MCS) over Houston, Texas, on 31 October 2005. Previous research examining only the mature stage of an MCS showed that the overwhelming majority of VHF lightning sources occurred in the convective region with a lightning pathway extending rearward and descending in altitude into the stratiform region. This descending pathway was most likely associated with small, charged ice particles advected from the convective line. Unlike previous research, the lightning pathway observed during the evolution of the MCS on 31 October 2005 initially extended rearward 40 km at a nearly constant height of 9–10 km. In less than an hour, the lightning pathway evolved into a sloped pathway, similar to that found by previous research, with a horizontal extent between 50 to 60 km and downward descent of 4 to 5 km. During the lightning pathway evolution, radar analysis showed an increase of reflectivity in the midlevels of the stratiform region, consistent with increased depositional and aggregational growth above the melting layer. Broadening (increased range of values) of radar reflectivity above the melting layer was most likely an indication of a strengthening mesoscale updraft in the stratiform region. This strengthening updraft may have contributed to an increase in the growth of the small, charge carrying ice crystals giving them a greater fall speed. In addition, the mesoscale updraft may have promoted an environment conducive to local stratiform region charge generation in the mixed phase region just above the melting layer.

Evolution of radar reflectivity and total lightning characteristics of the 21 April 2006 mesoscale convective system over Texas

On 21 April 2006 a mesoscale convective system (MCS) passed within range of the Houston (KHGX) operational Weather Surveillance Radar — 1988 Doppler (WSR-88D, S-band) and the Houston Lightning Detection and Ranging (LDAR) network, which measures the time and three-dimensional location of total, or both intracloud (IC) and cloud-to-ground (CG), lightning. This study documents the evolution of total lightning and radar reflectivity for the 21 April 2006 MCS over Texas, with emphasis on the stratiform region and those processes in the convection region that likely influence stratiform region development. As the MCS traverses the LDAR network, the system slowly matures with a weakening convective line and a developing stratiform region and radar bright band. The area of stratiform precipitation increases by an order of magnitude and mean stratiform radar reflectivity increases by 7–8 dB in the radar bright band and mixed-phase zone (0° to − 10 °C) just above it. As the stratiform region matures, the total lightning pathway slopes rearward and downward from the back of the convective line and into the stratiform region. At early times, the pathway extends horizontally rearward 40 to 50 km into the stratiform region at an altitude of 10 to 12 km. Near the end of the analysis time period, the total lightning pathway slopes rearward 40 km and downward 6 km through the transition zone before extending 40 to 50 km in the stratiform region at an altitude of 5 to 7 km. The sloping pathway likely results from charged ice particles advected from the convective line by storm relative front to rear flow while the level pathway extending further into the stratiform region is likely caused by both charge advection and local in-situ charging. As the stratiform region matures, the stratiform region total lightning flash rate increases and total lightning heights decrease. The percentage of stratiform total lightning flashes originating in the stratiform region increases significantly from 10% to 45%. The number of positive CG flashes in the stratiform region also increases with 73% originating in the convective or transition regions. Both in-situ charging mechanisms created by the development of the mesoscale updraft and charge advection by the front to rear flow likely contribute to the increased electrification and total lightning production of the stratiform region.

Lightning Charge Retrievals: Dimensional Reduction, LDAR Constraints, and a First Comparison with LIS Satellite Data

Abstract A “dimensional reduction” (“DR”) method is introduced for analyzing lightning field changes (ΔEs) whereby the number of unknowns in a discrete two-charge model is reduced from the standard eight (x, y, z, Q, x′, y′, z′, Q′) to just four (x, y, z, Q). The four unknowns (x, y, z, Q) are found by performing a numerical minimization of a chi-square function. At each step of the minimization, an overdetermined fixed matrix (OFM) method is used to immediately retrieve the best “residual source” (x′, y′, z′, Q′), given the values of (x, y, z, Q). In this way, all eight parameters (x, y, z, Q, x′, y′, z′, Q′) are found, yet a numerical search of only four parameters (x, y, z, Q) is required. The DR method has been used to analyze lightning-caused ΔEs derived from multiple ground-based electric field measurements at the NASA Kennedy Space Center (KSC) and U.S. Air Force Eastern Range (ER). The accuracy of the DR method has been assessed by comparing retrievals with data provided by the lightning detection and ranging (LDAR) system at the KSC-ER, and from least squares error estimation theory, and the method is shown to be a useful “stand alone” charge retrieval tool. Since more than one charge distribution describes a finite set of ΔEs (i.e., solutions are nonunique), and since there can be appreciable differences in the physical characteristics of these solutions, not all DR solutions are physically acceptable. Hence, an alternative and more accurate method of analysis is introduced that uses LDAR data to constrain the geometry of the charge solutions, thereby removing physically unacceptable retrievals. The charge solutions derived from this method are shown to compare well with independent satellite- and ground-based observations of lightning in several Florida storms.

Cross-sensor comparison of the Lightning Imaging Sensor (LIS)

The mapping of the lightning optical pulse detected by the Lightning Imaging Sensor (LIS) is compared with the radiation sources by Lightning Detection and Ranging (LDAR) at KSC and the National Lightning Detection Network (NLDN). Flash-based comparisons are done for the 15 August 1998 case including 122 flashes. The temporal and spatial differences are examined. For ground flash, the time difference of the first LDAR source and first LIS event has a mean of 0.23 s and the total duration of the flash has a mean of 0.28 s, compared to 0.56 s by LDAR. The LIS records the subsequent return stroke or K-change component. For cloud flash, the time difference has a mean of 0.2 s and the total duration of the flash has a mean of 0.38 s, compared to 0.44 s by LDAR. The LIS also records cloud flashes at higher altitude. The location differences are about 4 km for cloud flash and 12 km for ground flash.

A diagnostic analysis of the Kennedy Space Center LDAR network: 1. Data characteristics

An analytic framework is developed in which to analyze climatological VHF (66 MHz) radiation measurements taken by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network. A 19 month noise‐filtered sample of LDAR observations is examined using this framework. It is found that the climatological impulsive VHF source density as observed by LDAR falls off ∼10 dB every 71 km of ground range away from the network centroid (a 31 km e‐folding scale). The underlying vertical distribution of impulsive VHF sources is approximately normally distributed with a mean altitude of 9 km and a standard deviation of 2.7 km; this implies that the loss of below‐horizon sources has a negligible effect on column‐integrated source densities within a 200 km ground range. At medium to far ranges, location errors are primarily radial and have a slightly asymmetric distribution whose standard deviation increases as r2. Error moments estimated from observed lightning are significantly higher than those from aircraft‐based signal generator or analytic estimates. LDAR bulk flash detection efficiency is predicted to be above 90% to 94–113 km range from the network centroid and to fall below 10% at ranges greater than 200–240 km.

A diagnostic analysis of the Kennedy Space Center LDAR network: 2. Cross‐sensor studies

Range dependencies in total (intracloud and cloud to ground) lightning observed by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network are established through cross comparison with other lightning sensors. Using total lightning observed from space by the Lightning Imaging Sensor (LIS), LDAR flash detection efficiency is shown to remain above 90% out to 90–100 km range, and to be below 25% at 200 km range. LDAR VHF source location error distributions are also determined as a function of range and are found to be asymmetric with standard deviation increasing roughly as r2. Range normalization schemes for total VHF source density are tested and shown to yield significant improvements in correlation with National Lightning Detection Network (NLDN) ground flash density at hourly, daily, monthly, and climatological timescales (up to 50% over uncorrected source densities using an exponential‐in‐range correction factor with 40–50 km e‐folding scale).

Statistical estimation of locations of lightning events

In this paper, a statistical approach to the retrieval of lightning locations is proposed for the first time. This novel approach views the errors of the time measurements as random variables rather than unknown numbers. The unknown location (x, y, z) as well as the standard deviation σ of the errors are treated as unknown parameters of a statistical model and are estimated using the maximum likelihood estimation (MLE) technique. On the basis of Monte Carlo simulations these statistical estimators are compared with the least squares estimators (LSE), as well as the solutions of the system of linear equations proposed by Koshak and Solakiewicz [1996]. Although the method is general, the Lightning Detection and Ranging (LDAR) system currently used at the Kennedy Space Center is chosen as a model for simulations. Simulations show that the MLE always gives better precision than the LSE technique. Also, it is demonstrated that if the time measurements are fairly accurate and a thunderstorm takes place in the neighborhood of the measuring sites (the distance is less than 80 km), the MLE significantly improves the accuracy of the solutions of the system of linear equations.

Lightning source locations from VHF radiation data for a flash at Kennedy Space Center

A comprehensive study of a three‐stroke lightning flash that struck the 150‐m weather tower at Kennedy Space Center (KSC) was recently reported by Uman et al. (1978). Here we supplement that study by presenting three‐dimensional locations of the sources of VHF (30–50 MHz) radiation generated by the KSC flash. These locations were obtained by measuring with the KSC lightning detection and ranging (LDAR) system the difference in the time of arrival (DTOA) of radiated pulses at four ground stations and then by calculating the locations from the DTOA data. The DTOA data were obtained via a newly developed computer‐implemented multiple time series analysis which uses cross‐correlation and pattern recognition techniques. About 48,000 sequential VHF sources were located during the flash. We compared these locations with correlated wide band electric field records to provide a better understanding of the physics of the following discharge phases: preliminary breakdown, stepped leader, return strokes, dart leader, stepped‐dart leader, J changes, solitary pulses, and intracloud discharge. Some of the more interesting new results obtained follow: before the stepped leader preceding the first return stroke, there was a primarily vertical discharge in the cloud between a height of 7.2 and 5.1 km. This preliminary breakdown has uniquely different VHF characteristics from those of the stepped leader which followed and was not accompanied by significant electric field change as was the stepped leader. The stepped leader propagated downward from the bottom of the preliminary breakdown. The only significant VHF radiation during the dart leader preceding the second return stroke came from within the cloud. The third return stroke was initiated by a stepped leader which propagated from about 8 km height, the bottom of the previous J process, down to about 3.3 km where it apparently joined the channel of the previous return stroke and became a dart leader. The VHF source locations during the two J changes were associated with the lowering of about 3 C of negative charge at velocities of about 2×105 m/s along primarily vertical paths from a height of about 13.7 to about 7.9 km for the first J change and from 11.2 to 7.3 km for the second J change. During the second J change and before and during the intracloud discharge following the three strokes to ground, discharges which appear on the VHF record as isolated or solitary pulses propagated upward 3–8 km at velocities of the order of 107 m/s. The intracloud discharge had a length of more than 10 km and was oriented primarily vertically. The VHF radiation during the intracloud discharge exhibited the three phases reported in the literature for cloud discharges not associated with ground discharges. The highest level of VHF activity during the flash occurred during 4.3 ms just after a 2.4 ms quiet period following the first return stroke. These VHF sources were located primarily at the top of the return stroke channel.